MEMS pressure sensors are generally known. One type of pressure sensor is a differential pressure sensor which includes a pressure sensing element made of silicon which is anodically bonded to a glass pedestal, and the glass pedestal is mounted to a housing substrate using an adhesive. Many differential pressure sensors are used in applications in which the sensors are exposed to varying temperatures. This causes the sensing element, the glass pedestal, the adhesive, and the housing substrate to expand and contract in response to the temperature changes.
The pressure sensing element includes four piezoresistors or resistors positioned in what is known as a “Wheatstone Bridge” configuration. The adhesive expands and contracts at a different rate in relation to the pressure sensing element, which can cause stress to be applied to the resistors, affecting the pressure reading detected by the pressure sensing element. The glass pedestal is incorporated between the pressure sensing element and the adhesive such that the stresses resulting from the difference in thermal expansion between the pressure sensing element and the adhesive are isolated by the glass pedestal. The glass pedestal and the pressure sensing element have slightly different coefficients of thermal expansion, and therefore expand and contract at a lower different rate when exposed to varying temperatures. The glass pedestal essentially acts as a buffer to isolate the stresses resulting from the different expansion and contraction rates between the glass pedestal and the adhesive.
An example of the pressure sensor discussed above is shown in FIGS. 1-2B generally at 10. The sensor 10 includes a pressure sensing element 12, a glass pedestal 14, an adhesive 16, and a housing substrate 18. The pressure sensing element 12 shown in FIG. 1 is made from silicon, and is anodically bonded to the glass pedestal 14. The adhesive 16 is used to bond the glass pedestal 14 to the housing substrate 18.
Formed as part of the housing substrate 18 is a first aperture 20, and formed as part of the glass pedestal 14 is a second aperture 22, which is in substantial alignment with the first aperture 20. The second aperture 22 is in fluid communication with a cavity, shown generally at 24, where the cavity 24 is formed as part of the pressure sensing element 12. The pressure sensing element 12 includes four angular inner surfaces, where only a first angular inner surface 26 and a second angular inner surface 28 are depicted in FIG. 1, because FIG. 1 is a cross-sectional view. Each of the four angular inner surfaces terminates into a bottom surface 30, which is part of a diaphragm 32. The pressure sensing element 12 also includes a top surface 34, and there is a picture frame transducer or picture frame Wheatstone bridge 36 doped onto the top surface 34 of the pressure sensing element 12. At least a thermal oxide layer and passivation layers are formed to protect the circuitry. The picture frame Wheatstone bridge 36 is formed by four p− piezoresistors 36A-36D as shown in FIG. 2B. The four piezoresistors 36A-36D may also be formed as a distributed Wheatstone bridge 38A-38D as shown in FIG. 3 for pressure sensing.
The diaphragm 32 is relatively thin, and the thickness of the diaphragm 32 depends upon the pressure range. The diaphragm 32 deflects upwardly and downwardly in response to pressure applied to the bottom surface 30, and the top surface 34 of the diaphragm 32. The pressure in the cavity 24 changes as a result of a pressure change of fluid flowing into and out of the apertures 20,22.
The deflections in the top surface 34 also deform the picture frame Wheatstone bridge 36, which is doped onto the top surface 34 of the pressure sensing element 12. The pressure sensing element 12 is made of a single crystalline silicon (Si). On the top of the pressure sensing element 12, four p− piezoresistors 36A-36D are formed and connected to each other by p+ interconnectors 40 to form the picture frame Wheatstone bridge 36 for pressure sensing as shown in FIGS. 2A-2B.
Merriam-Webster's Collegiate Dictionary 11th Edition defines a Wheatstone bridge as an electrical bridge consisting of two branches of a parallel circuit joined by a galvanometer and used for determining the value of an unknown resistance in one of the branches. As used herein, the term Wheatstone bridge refers to the circuit topology shown in FIG. 2A-2B, namely the parallel connection of two series-connected resistors.
FIGS. 2A-2B represent a top view of the piezoresistive pressure sensing element 12 with the picture frame Wheatstone bridge 36, which is doped on the diaphragm 32. The diaphragm 32 has dimensions of 780 μm×780 μm. The thickness of the diaphragm 32 is generally in the range of about 5 μm to 20 μm, and as shown in FIGS. 2A-2B, is about 9 μm. The picture frame Wheatstone bridge 36 is processed using conventional techniques to form four resistors 36A-36D on the top surface of the pressure sensing element 12. The resistors 36A-36D are formed of a p− material, embodiments of which are well-known to those of ordinary skill in the semiconductor art. Electrical interconnects 40 made of p+material connected to the bottom of bond pads 42A-42D are also formed on the top surface 34 of the pressure sensing element 12. Each interconnect 40 provides an electrical connection between two resistors in order to connect the resistors to each other to form a piezoresistive Wheatstone bridge circuit.
The four interconnects 40 are shown as part of the pressure sensing element 12. Each interconnect 40 extends outwardly from a point or node 44 between two of the four resistors 36 next to each other, and connects to the bottom of a metal bond pad 42. Each bond pad 42 is located near a side 46 of the top surface 34 of the pressure sensing element 12. Each interconnect 40 thus terminates at and connects to a bond pad 42.
FIG. 2A also shows an orientation fiducial 48 on the top surface 34. The fiducial 48 is a visually perceptible symbol or icon the function of which is simply to enable the orientation of the pressure sensing element 12.
Each bond pad 42 has a different label or name that indicates its purpose. The first bond pad 42A and the second bond pad 42B receive an input or supply voltage for the Wheatstone bridge circuit. Those two bond pads 42A,42B are denominated as Vp and Vn, respectively. The other two bond pads 42C,42D are output signal nodes denominated as Sp and Sn, respectively.
Many attempts have been made to simplify the construction of this type of pressure sensor 10 by eliminating the glass pedestal 14, and directly connecting the pressure sensing element 12 to the housing substrate 18 with the adhesive 16. However, the difference in thermal expansion between the adhesive 16 and the pressure sensing element 12 has resulted in unwanted stresses being applied to the pressure sensing element 12, which then disrupt each of the resistors 36A-36D, causing an inaccurate pressure reading by the pressure sensing element 12.
More particularly, both experimental measurement and computer simulations of the structure depicted in FIGS. 1-2B show that connecting the pressure sensing element 12 directly to the housing substrate 18 creates offset voltage output and its variation over an operating temperature range due to asymmetrical thermal stresses on the resistors 36A-36D. Elimination of the glass pedestal 14 causes one of the resistors 36A through 36D to deform or stressed, or to change its resistance value asymmetrically with respect to the other resistors leading to an offset voltage output variation in an operating temperature range in the output of the pressure sensing element 12.
The offset voltage output variation over an operating temperature is called temperature coefficient of offset voltage output (TCO) and defined as follows:TCO=(Vo at 150° C.−Vo at −40° C.)/190° C.Where Vo at 150° C.: offset voltage output at 150° C. without pressure applied
Vo at −40° C.: offset voltage output at −40° C. without pressure applied
The pressure sensing element 12 is commonly used with an application-specific integrated circuit (ASIC). The ASIC is, among other things, used for amplifying and calibrating the signal received from the pressure sensing element 12. It is desirable to keep the TCO between −50 uV/° C. and 50 uV/° C. so the ASIC is better able to handle any thermal noise.
The high TCO is difficult for an ASIC to compensate, especially when the adhesive 16 is not symmetrically dispensed. If the adhesive is not symmetrically dispensed, this can further reduce the accuracy of the sensor. The stress difference in the X and Y directions on each of the four resistors is amplified, thus the offset voltage outputs increase, as well as the TCO. That is why the glass pedestal 14 shown in FIG. 1 is used to isolate the thermal stresses. In order to reduce cost and simplify the manufacturing process, it is desirable to eliminate the glass pedestal. A pressure sensing element without a glass pedestal also improves wire bonding stability and reliability. Therefore, there is a need for a type of pressure sensor which does not have a glass pedestal, but has a low TCO noise.